The quest for robust electronic devices singles out diamond as a most favorable candidate, however its doping is still problematic. The unique p-type surface conductivity induced by air exposure of hydrogen terminated diamond discovered by Landstrass and Ravi in 1989 (Landstrass and Ravi, 1989), opened up exceptional applications of diamond-based devices, such as high-power high-frequency devices (Isberg et al., 2002; Taniuchi et al., 2001; Wort and Balmer, 2008), field effect transistors (Kasu et al., 2005; Russell et al., 2012), nanoscale planar doped electronic devices (Sussmann, 2009; Geisler and Hugel, 2010), electrochemical electrodes (Poh et al., 2004; Christiaens et al., 2006), and biological electrodes sensors (Yang et al., 2002; Lud et al., 2006; Dankerl et al., 2011).
The two dimensional (2D) high p-type surface conductivity of diamond is induced by exposing a hydrogen terminated diamond surfaces (diamond:H) to high work function adsorbate molecules, leading to the so-called transfer doping (TD) of the diamond near surface (Strobel et al., 2004; Chakrapani et al., 2007). As explained by the electrochemical TD model (Maier et al., 2000), electrons transfer from the diamond valence band maximum (VBM) to lowest unoccupied molecular orbital of the adsorbate layer, leaving behind a hole accumulation layer. This results in an upward band bending and in the formation of a two dimensional hole layer extending to only a couple of nanometers from the surface into the diamond, exhibiting 2D quantization (Bolker et al., 2011).
Several potentially suitable molecules with high electron affinity and high work function values, have been examined as possible candidates for the surface TD of diamond:H (Ristein and Riedel, 2004; Chen et al., 2009). The first, and most widely studied adsorbate molecule, which has yielded the highest areal hole density measured so far (5×1013 cm−2), is the aqueous (H2O) layer adsorbate (Sauerer et al., 2001). TD in this case is simply achieved by exposure of the diamond:H surface to humid air. However, the required water coverage is sensitive to environment humidity and, in particular, depends on temperature which results in gradual loss of H2O during warming up, until a complete desorption occurs at 300° C. (Laikhtman et al., 2004). Hence the thus obtained p-type conductive layer suffers from low controllability and lack of thermal stability.
It is most desirable to find a more stable substitute to water for efficient TD of diamond. Several adsorbate molecules fulfilling diamond:H TD conditions have been studied, including fullerenes (C60), fluorinated Fullerenes C60Fx (x=18, 36, 48) (Strobel et al., 2006). Tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) (Qi et al., 2007) and, most recently, MoO3 (Russell et al., 2013). The thermal stability of these has, however, either not been investigated or was found not to be stable at elevated temperatures. Furthermore, all cases studied so far have exhibited lower conductivity values than that of water transfer doped diamond. Hence the need to find a material for TD of diamond which does not suffer from the above limitations.